Recently a low cost fluidized bed process for producing ethylene polymers having a density of from 0.86 g/cm.sup.3 to 0.96 g/cm.sup.3 was introduced into the marketplace. This process, which met with immediate success and has now become the industry standard, comprises continuously copolymerizing ethylene with one or more higher alpha olefin monomers, usually butene, by means of a catalyst composition prepared by (1) forming a precursor composition from a magnesium compound, titanium compound and electron donor compound, (2) diluting said precursor composition with an inert carrier material, and (3) activating the precursor composition with an organoaluminum compound. Such process is more completely described in U.S. Pat. Nos. 4,302,565, 4,302,566 and 4,303,771, and European patent publication 0 120 503.
While this process has substantially reduced the cost of producing ethylene polymers, the cost still remains higher than desired because of the relatively high cost of the higher alpha olefin comonomer employed in the process vis-a-vis the cost of ethylene, particularly in those areas of the world where the disparity in price between such comonomer and ethylene is substantial. In view of this, it would be desirable to reduce the cost of producing such comonomer in order to further reduce the cost of the process and the polymers produced thereby.
One method suggested for producing higher alpha olefin comonomer comprises oligomerizing ethylene to higher alpha olefins, e.g., dimerizing ethylene to produce butene. Such dimerization has only infrequently been attempted in situ during the polymerization of ethylene, however, because of the difficulty of producing a compatible dimerization and polymerization system, i.e., one wherein the dimerization catalyst and polymerization catalyst operate under the same reaction conditions and do not interfere with each other chemically. Such in situ dimerization would be desirable, however, as a means of eliminating one of the comonomers employed, simplifying the polymerization process, and reducing the overall cost of the polymers produced.
One means of simultaneously dimerizing ethylene in situ and copolymerizing ethylene with the dimerized product has been suggested in U.S. Pat. No. 4,133,944. This method employs a transition metal catalyst, such as a Ziegler-type catalyst, together with a dimerization catalyst, such as a titanium alkoxide, and an alkyaluminum compound, for this purpose and requires temperatures of from 100.degree. C. to 350.degree. C. and pressures of from 300-1000 kgm/cm.sup.2.
While U.S. Pat. No. 3,526,616 suggests that simultaneous in situ dimerization and copolymerization of ethylene can be effected by means of a similar catalyst under milder temperature and pressure conditions, this patent requires that the transition metal compound employed be reacted with a bivalent metal hydroxychloride support in order to effect dimerization under such conditions.
The simultaneous in situ dimerization and copolymerization of ethylene with similar catalysts under mild temperature and pressure conditions has also been taught by David L. Beach and Yury V. Kissin (Dual Functional Catalysis for Ethylene Polymerization to Branched Polyethylene. I. Evaluation of Catalytic Systems, Journal of Polymer Science: Polymer Chemistry Edition, Vol. 22, 3027-3042 (1984)). This reference further teaches that while solubilized titanium alkoxides produce fairly active dimerization catalysts, insolubilized compounds make ineffective catalysts. Thus, this system depends upon the presence of a solvent for its effectiveness, and suggests that such system is not effective in a gas phase fluid bed process.